Light emitting diodes, commonly referred to as "LED's", are semiconductor devices which convert electrical energy into light. As is known to those familiar with semiconducting materials, diodes formed from certain types of materials will produce energy in the form of light when current passes across the p-n junction in such a semiconducting diode. When current passes across a diode's junction, electronic events occur that are referred to as "recombinations," and in which electrons in the semiconductor combine with vacant energy level positions, referred to as "holes," in the semiconductor. These recombination events are typically accompanied by the movement of an electron from a higher energy level to a lower one in the semiconductor material. The energy difference between the energy levels determines the amount of energy that is be given off. When the energy is given off as light (i.e. as a photon), the difference in energy levels results in a particular corresponding wavelength of light being emitted. Because the positions of various available energy levels are a fundamental characteristic of any particular element or compound, the color of light that can be produced by an LED is primarily determined by the semiconductor material in which the recombination is taking place. Additionally, the presence in the semiconductor material of added dopant ions, which are referred to as either "donors" because they provide extra electrons, or as "acceptors" because they provide additional holes, results in the presence of additional energy levels in the semiconductor material between which electrons can move. This in turn provides different amounts of energy that are given off by the available transitions and provides other characteristic wavelengths of light energy given off by these additionally available transitions.
Because of this relationship between energy and wavelength--which in the visible portion of the electromagnetic spectrum represents the color of the light--blue light can only be produced by a semiconductor material having a band gap larger than 2.6 electron volts (eV). The "band gap" refers to the basic energy transition in a semiconductor between a higher or "conduction" band energy level and a more regularly populated lower or "valence" energy band level. For example, materials such as gallium phosphide (GaP) or gallium arsenide (GaAs) cannot produce blue light because the band gaps are on the order of about 2.26 eV or less. Instead, a blue light emitting solid state diode must be formed from a semiconductor with a relatively large band gap such as gallium nitride (GaN), zinc sulfide (ZnS), zinc selenide (ZnSe) and alpha silicon carbide (also characterized as "hexagonal" or "6H silicon carbide," among other designations. Accordingly, a number of investigators have attempted to produce blue light emitting diodes using alpha silicon carbide.
Nevertheless, silicon carbide has not presently reached the full commercial position in the manufacture of electronic devices, including light emitting diodes, that would be expected on the basis of its otherwise excellent semiconductor properties and its potential for producing blue LED's. For example, in addition to its wide band gap, silicon carbide has a high thermal conductivity, a high saturated electron drift velocity, and a high breakdown electric field. All of these are desirable properties in semiconductor devices including LED's. The failure of silicon carbide LED's to reach commercial success appears to be the result of the difficulties encountered in working with silicon carbide. In particular, high process temperatures are required, good starting materials are typically difficult to obtain, particular doping techniques have heretofore been difficult to accomplish, and perhaps most importantly, silicon carbide crystallizes in over 150 polytypes, many of which are separated by very small thermodynamic differences.
Accordingly, the goal of controlling the growth of single crystals or monocrystalline thin films of silicon carbide which are of sufficient quality to make electronic devices such as diodes practical, useful, and commercially viable, has eluded researchers in spite of years of diligent effort, much of which is reflected in both the patent and nonpatent literature.
Recently, however, a number of developments have been accomplished which offer the ability to grow large single crystals of device quality silicon carbide, to grow thin films of device quality silicon carbide, to successfully etch silicon carbide, and to introduce dopants into silicon carbide, all steps that are typically required in the manufacture of LED's and other electronic devices. These developments are the subject of co-pending patent applications that have been assigned or exclusively licensed to the common assignee of the present invention and which are incorporated entirely herein by reference. In addition to the application mentioned earlier, these include Davis et al, "Growth of Beta-SiC Thins Films and Semiconductor Devices Fabricated Thereon," Ser. No. 113,921, Filed Oct. 26, 1987; Davis et al, "Homoepitaxial Growth of Alpha-SiC Thin Films and Semiconductor Devices Fabricated Thereon," Ser. No. 113,573, Filed Oct. 26, 1987; Davis et al, "Sublimation of Silicon Carbide to Produce Large, Device Quality Single Crystals of Silicon Carbide," Ser. No. 113,565, Filed Oct. 26, 1987; Palmour, "Dry Etching of Silicon Carbide, Ser. No. 116,467, filed Nov. 3, 1989; and Edmond et al, "Implantation and Electrical Activation of Dopants Into Monocrystalline Silicon Carbide," Ser. No. 113,561, Filed Oct. 26, 1987.
As set forth in some detail in the Edmond '293 application, a number of different doping techniques and basic device structures are used to produce light of approximately 424-428 nanometers (nm), light of approximately 455-460 nm, and light of approximately 465-470 nm in diodes formed from silicon carbide. Although describing the visible colors of these wavelengths is somewhat of a generalization, the 424-428 nm light has a characteristic violet color, the 455-460 nm transition gives a more medium blue color, and the 465-470 nm transition gives a characteristic light blue color.
As further set forth in the Edmond '293 application, one usual goal in producing LED's is to obtain as much emitted light as possible. This is addressed by a number of techniques familiar to those in the art, some of which are: injecting as much current as possible across the p-n junction; having the greatest possible dopant population in the emitting layer; obtaining the greatest possible efficiency in producing recombination events; and using a physical structure, including the optical characteristics of the semiconductor material itself that enhances the visible light obtained from the diode. With regard to this last characteristic, a transparent semiconductor material will often be the most desireable.
Using these considerations, one of the more desirable and efficient transitions in silicon carbide is that between an impurity band of nitrogen (donor) below the conduction band and an impurity band of aluminum (acceptor) above the valence band. This transition is especially favorable when combined with a physical structure that encourages most of the current passing across the junction to be p-type current; i.e. the flow of holes across the junction and into the n-type material. As is known to those familiar with silicon carbide, the donor band of nitrogen is approximately 0.075 eV below the conduction band of silicon carbide, while the acceptor band of aluminum is approximately 0.22 eV above the valence band. The resulting transition is on the order of about 2.62 eV and emits a photon having a wavelength between about 465 nm and 470 nm and with a characteristic blue color.
Furthermore, in order to produce this transition, one of the portions of the diode must be doped with both donor and acceptor dopants, but with one dopant predominating over the other to give a distinct p or n electrical characteristic to the material This technique is referred to as "compensation," and the resulting portion of semiconductor material is referred to as being "compensated." For example, in order to use hole current to produce blue light in a silicon carbide LED--another favorable technique in particular applications--the portion of the diode which is n-type must be doped with both donor (often nitrogen) and acceptor (often aluminum) dopants, with the nitrogen predominating, to give an overall n-type characteristic even with the acceptor atoms present.
Certain problems arise, however, in attempting to form LED's that have these characteristics. For example, where a p-type substrate is desirably or necessarily used (depending upon the manufacturing technique used or the device that may be desired) it will have a rather high resistivity. This results from the well-known facts that the mobility of holes is one-sixth that of electrons, and that typically less than two percent of acceptor atoms are ionized (i.e. able to act as charge carriers) at room temperature. These characteristics result in a higher resistance in forward bias for p-substrate diodes, which is a less desirable trait for a diode.
One attempted manner of addressing the resistivity problem is to increase the hole concentration in the p-type substrate. The addition of the extra dopant required to increase the hole concentration, however, tends to make the crystal opaque and reduces the emitted light that can be observed. Conversely, by keeping the dopant concentration lower, the crystal will be more transparent, but at the cost of an undesirably high resistivity.
The problems associated with high resistivity substrates can also be addressed mechanically, for example by avoiding using the substrate portion as a conductor in the diode. U.S. Pat. No. 4,531,142 discusses such a mechanical technique. This is an extremely difficult manufacturing technique, however, as reflected in the low availability and high cost of such diodes. Another solution is to use a relatively large ohmic contact to the p+ layer so as to increase the current across the junction. The practical effect, however, is to block light from being emitted from the p+ layer by the presence of the ohmic contact.
In contrast the co-pending and incorporated Edmond '293 application teaches a number of solutions to these problems and in particular discusses the successful use of chemical vapor deposition as set forth in the similarly copending and incorporated Davis '573 application in order to successfully produce such diodes.
Once such diodes are shown to be practical and efficient, however, interest and need arises for having them manufactured on a commercial scale. For example, in one of the most effective LED's described by the Edmond '293 application, the diode consists of an n-type substrate, an n-compensated epitaxial layer and a p+-epitaxial layer. Under a forward bias, hole current injected from the p+ to the n-compensated layer is the predominating current in this diode. As discussed earlier and in the Edmond '293 application, the generally higher resistivity of the top p+ layer makes it more difficult to get an appropriate amount of current spreading, which is exhibited as a corresponding lack of uniformity in the light generated in that layer.
Because the blue light generated by diodes formed of alpha silicon carbide is commercially desirable, however, there exists the additional need for developing a method and structure for commercially producing and packaging such diodes and which will have advantages for other similar diodes as well. As is known to those familiar in the industry, in order to be useful such a diode has to be mounted and it is most useful if such a diode can be conventionally mounted using techniques such a conductive epoxy and reflective cups. Similarly, the nature and position of the ohmic contacts and the crystal structure itself should also be designed for commercial manufacture and the techniques used should avoid damaging the junctions during the manufacturing process. Finally, such a technique should be appropriate for multiple or mass production. To date, appropriate commercial applications of this type are rarely seen in the pertinent art.